It is well know to produce endless lengths of welded steel tubing from strip stock into continuously galvanized tubing by providing a zinc coating on the exterior surface as taught, for example, in U.S. Pat. Nos. 3,122,114, 3,230,615 and 3,561,096, which are owned by the assignee of this patent application.
It is also desirable to form continuous galvanized tubing from galvanized strip stock to provide tubing which is galvanized on both the interior and exterior surfaces in order to increase longevity of the tubing. In a method for producing such galvanized tubing, a metallizing gun, known in the art for spraying melted, finely particulated zinc (e.g. to galvanize threaded sections of a workpiece) is used to complete the galvanization of the exterior surface, after the seam of the tubing is scarfed to remove the bead resulting from welding.
A problem with galvanized tubing produced by prior art methods in which zinc coating is sprayed onto the seam area with a metallizing gun is that the zinc coating of the seam is susceptible to flaking and cracking from stress when the tubing is bent by workers at the site of installation, presumably because the sprayed-on zinc coating does not form any intermetallic compound with the steel substrate at the scarfed area. Consequently, premature corrosion of the tubing at points where the sprayed-on zinc coating has cracked or flaked off is a serious disadvantage of such tubing. It would therefore be very desirable to provide an improved method of manufacturing continuous lengths of tubing having both a galvanized interior surface and a continuous, integral, galvanized exterior surface, which tubing may be bent without exhibiting the above-discussed problem.